Art in Focus of Synchrotron Microscopes
X-Ray and Infrared Microspectroscopy Reveal Art Chemistry
- Fig. 1: Schematic view of the main questions and main strategies for chemical characterization of artefacts.
- Fig. 2: The analytical and the structural investigations carried out on double cross-section sample sampling from a fragment of Ming Qinghua porcelain.
- Fig. 3: Epsomite crystals forming at the surface of a 1964 chrome yellow paint model sample. The red and green maps are sulphur µXRF maps obtained at the two energies E1 and E2. The blue map is Mg. The orange and green XANES spectra are averaged spectra acquired in the orange (8 points) and green (7 points) areas respectively and are compared with spectra of reference sulphate compounds.
The X-ray and infrared microscopes at ID21 beamline, European Synchrotron Radiation Facility, are regularly used to get insight into artistic materials via elemental, chemical and phase 2D maps acquired on micrometric fragments. The compounds constituting the artistic matter (original ingredients, degradation products, conservation treatments, pollutants…) can be identified and localized at the micron scale. Information can thus be obtained about manufacturing recipes and degradation mechanisms.
When Roentgen reported the discovery of X-rays in 1895, he also pioneered their application for X-ray radiography of paintings . Since then, other X-ray based techniques, such as X-ray fluorescence (XRF), X-ray diffraction (XRD) or X-ray absorption spectroscopy (XAS) have been developed and successfully applied to study our Cultural Heritage. Considering the high complexity and heterogeneity of artistic materials, from the nm to the cm scale, or even the m scale, imaging techniques are ideal. Analyses can be carried out directly onto the object. For this purpose, many actions are led today to develop portable hyperspectral imaging instruments, allowing on-site (museums, archaeological sites, historical buildings) analyses. Alternatively, or complementarily, micro-analyses can be carried out on tiny fragments to investigate the chemical composition at the (sub)-micron scale. As a complement to optical, electron- and ion-based microscopes, synchrotron radiation (SR)-based X-ray microscopes offer many advantages . The high brightness, collimation and partial coherence of the beam convert into a small (down to few tens of nm) and intense beam, even in monochromatic mode. Compared to standard laboratory instruments, SR-analyses can be carried out with a higher lateral resolution, faster (higher number of points, for 2D and 3D scanning maps) and with increased signal to noise ratio. Besides, the very large accessible energy range and energy tunability offer spectroscopy capabilities, from X-rays to UV/visible and infrared domains and consequently complementarity chemical contrasts. As an example, the ID21 beamline at the European Synchrotron Radiation Facility (ESRF) offers four end-stations (three in scanning mode, exploiting µXRF, µXAS, µXRD and infrared micro-spectroscopy (µFTIR) and one in full-field mode dedicated to XAS) and has an important activity in the field of cultural heritage .
Chemical Composition Analysis
As shown in figure 1, the two main objectives are usually either to understand how the object was created and retrieve original recipes, or to understand alteration and degradation phenomena and to assess restoration and preservation treatments.
However, the chemical composition, as probed today, is not necessarily identical to the one at the time of the creation (paintings may have dried, metallic sculptures may have corroded, etc.). Accordingly, understanding physical and chemical processes involved in the evolution of materials is necessary, not only to forecast the state of works of art in the future but also to “backcast” the state of materials years, decades or centuries ago. As a complement to the chemical characterization of artwork fragments, additional information can be obtained through the study of historical texts and iconography and the reconstruction of model samples. Testing various synthesis protocols and natural and artificial ageing conditions provide a benchmark to understand the entire life, from creation to degradation, of historical objects.
Artefacts may contain chemical signatures of recipes used for their manufacturing. The choice of ingredients (e.g. natural or synthetic), the use of chemical or physical processes (e.g. acid treatment, firing conditions, etc.) will have impact on the final elemental, chemical and structural properties of materials. The determination of these markers allows to follow technological evolution of recipes over time and space. As an example, blue pigments used to decor Chinese Qinghua porcelains of the Ming Dynasty (1368-1644) have been studied by combining µXRF, µXANES and µXRD . The objectives were to assess correlations between the color (intensity and tint) and some chemical markers. Samples were prepared as double cross-sections to preserve the fragile glaze surface (fig. 2 top). µXRF allowed studying the diffusion of pigment mixture elements into the glaze (fig. 2 middle). µXANES (in micro beam and full-field modes) tackled the speciation of Co, the main colorant element (fig. 2 bottom). µXRD revealed that the CoAl2O4 spinel was substituted and not a pure phase. The set of results showed the main role of the CoAl2O4 in the color (intensity and tint) and the minor role of the other transition elements (Mn and Fe). Mn has a marginal effect on the tint while iron play only a role in the glaze yellowing. However, these two elements provide additional useful information about the manufacturing processes.
For what concerns the preservation of Cultural Heritage, micro-chemical analyses aim at understanding degradation mechanisms, (e. g. corrosion in metallic artefacts, modification of colors in pigments , degradation of organic materials, being natural (e.g. wood ) or synthetic (e.g. plastics , modelling clay )) . These degradations usually result from the modification of the chemical composition, with oxidation/reduction reactions, dissolution/crystallisation, redistribution of ingredients, reactions with exogenous compounds such as pollutants, etc. The ID21 X-ray and infrared microscopes are regularly used to identify and localize the degradation products, both in historical and model samples. Model samples are usually artificially aged under many controlled conditions, allowing to selectively asses the role of various parameters: e.g. composition of the model sample, humidity, light(s), temperature, pollutants, etc. In terms of analytical techniques, it requires: i) a high chemical sensitivity (to distinguish subtle differences between safe and degraded matter), ii) a high lateral resolution (since the degradation layers are usually restricted to few microns), iii) a high throughput (to extend the range of assessed degradation triggers).
As an example, water sensitivity in manufactured artists oil paints and paintings presents significant challenges for cleaning modern oil paintings . In spite of the invention of synthetic paint media in the 20th C., many artists continued to use traditional oil paints, that offer a range of textural and color blending properties hardly achievable using fast drying acrylic paints. The accumulation of dirt on the surface of generally unvarnished modern paintings and consecutive visual changes require conservation treatments. Removal of surface dirt (in particular imbibed or polar material) is usually done using aqueous solutions. However, sensitivity to aqueous cleaning has been reported in many 20th C. paintings, with diverse behaviors: some colored oil paints are so sensitive that pigment is removed directly by touching the surface with a dampened swab; other paints show moderate or no sensitivity after repeated swab rolls of water on the surface. Recent research has revealed the formation of polar degradation products at the paint surface, deriving from pigment, binder or other additives, and responsible for the water sensitivity . SR-micro-analyses aim at identifying such products and at understanding their formation.
The example shown in figure 3 is a chrome yellow paint prepared in 1964, showing surface degradation and extreme sensitivity to water applied by swab, with pigment removed immediately. In this paint, BaSO4 and MgCO3 were introduced as extenders (Mg XRF map shown in blue in fig. 3). Micro-analyses reveal that MgCO3 has transformed into a water soluble magnesium sulphate heptahydrate (epsomite) on the paint surface. µXANES permits differentiating epsomite from BaSO4 and mapping their respective distribution by recording S µXRF maps at specific energies (E1 and E2 shown in fig. 3).
The on-going refurbishment of the ID21 beamline together with the upgrade of the ESRF (Extremely Brilliant Source) aims at extending chemical imaging capabilities, by increasing flux, detection efficient, energy range and by reducing beam size to 100nm. This will allow zooming further into artistic materials and switching from multi-spectral to hyper-spectral imaging.
Emeline Pouyet is thanked for her participation in the two studies presented here; all ESRF staff involved in the development and maintenance of instruments are thanked as well.
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M. Cotte1, A. Burnstock2, K. J. van den Berg3, P. Sciau4, T. Wang5
1 European Synchrotron Radiation Facility, Grenoble, France
2 Courtauld Institute of Art, Department of Conservation & Technology, London, United Kingdom
3 Cultural Heritage Agency of the Netherlands, Amsterdam, The Netherlands
4 CEMES, CNRS, Toulouse University, Toulouse, France
5 School of Materials Science and Engineering, Shaanxi University of Science and Technology, Shaanxi city, Xi'an province, China